1. The Field of the Invention
The present invention relates to metal lines used for electrically connecting devices on an integrated circuit and more specifically to the formation of aluminum lines in which the void formation therein is suppressed.
2. The Relevant Technology
Integrated circuits are manufactured by an elaborate process in which a variety of different electronic devices are integrally formed on a semiconductor substrate such as a small silicon wafer. In the context of this document, the term “semiconductor substrate” is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term “substrate” refers to any supporting structure including but not limited to the semiconductor substrates described above. The term semiconductor substrate is contemplated to include such structures as silicon-on-insulator and silicon-on-sapphire.
Conventional electronic devices include capacitors, resistors, transistors, diodes, and the like. In advanced manufacturing of integrated circuits, hundreds of thousands of electronic devices are formed on a single wafer. One of the final steps in the manufacture of integrated circuits is to form interconnect lines between a select number of the devices on the integrated circuit. In turn, the interconnect lines are connected to leads which can then be connected to other electrical systems. The interconnect lines in conjunction with the leads allow for an electrical current to be delivered to and from the electronic devices so that the integrated circuit can perform its intended function.
The interconnect lines generally comprise narrow lines of aluminum. Aluminum is typically used because it has a relatively low resistivity, good current carrying density, superior adhesion to silicon dioxide, and is available in high purity. Each of these properties is desirable in interconnect lines since they result in a quicker and more efficient electronic circuit.
The computer industry is constantly under market demand to increase the speed at which integrated circuits operate and to decrease the size of integrated circuits. To accomplish this task, the electronic devices on a silicon wafer are continually being increased in number and decreased in size. In turn, the size of the interconnect lines must also be decreased.
As the interconnect lines get smaller, however, a phenomenon referred to as “void formation” has been found to occur more frequently. In general, void formation is a process in which minute voids formed within the aluminum line coalesce on the boundaries of the aluminum line. As a result of the coalescing of the voids, the aluminum line begins to narrow at a specific location. If the aluminum line gets sufficiently narrow, the line can burn out so as to cause an open in the line. The open prevents the integrated circuit from operating in a proper manner.
Void formation is generally caused by either electromigration or stress migration. Electromigration occurs as an electrical current flows through an aluminum line. When a voltage is applied across an aluminum line, electrons begin to flow through the line. These electrons impart energy to the aluminum atoms sufficient to eject an aluminum atom from its lattice site. As the aluminum atom become mobile, it leaves behind a vacancy. In turn, the vacancy is also mobile since it can be filled by another aluminum atom which then opens a new vacancy. In the phenomenon of electromigration, the vacancies formed throughout the aluminum line tend to coalesce at the grain boundaries of the aluminum line, thereby forming voids that narrow the interconnect line as discussed above. Once the interconnect line is narrowed, the current density passing through that portion of the line is increased. As a result, the increased current density accelerates the process of electromigration, thereby continually narrowing the line until the line fails.
It is also thought that void formation occurs as a result of stress migration inherent in aluminum line deposition. The deposition of the aluminum lines is usually done at an elevated temperature. As the aluminum cools, the aluminum begins to contract. An insulation layer positioned under the aluminum layer, typically silicon dioxide, also contracts. The aluminum and the silicon dioxide have different coefficients of thermal expansion and contraction such that the two materials contract at different rates. This contraction sets an internal stress within the aluminum line. The same phenomenon can also occur when a subsequent layer is formed over the top of the aluminum line. It is theorized that the energy resulting from the induced stress within the aluminum causes displacement of the aluminum atoms and coalescence of the resulting vacancies.
FIG. 1 illustrates the problem of voids in exposed interconnect lines that are composed of aluminum. A semiconductor structure 10 is seen in FIG. 1 that includes a silicon substrate 12, a insulating layer 14, and interconnect lines 27 and 29 on insulating layer 14. Silicon substrate 12 has an active area therein to which a contact is made by a plug 15 having a liner 13 thereover. Plug 15 is preferably composed of aluminum or tungsten, and liner 13 is preferably composed of titanium nitride or a combination and titanium and titanium nitride. Upon insulating layer 14 is layer 17 composed of titanium and layer 19 composed of titanium aluminide. A layer 20 is composed aluminum and a layer 22 is composed of titanium nitride. Interconnect lines 27 and 29 have been patterned as illustrated.
A void 21 is seen in aluminum layer 20. The occurrence of a high mechanical stress field in aluminum layer 20 initiates the formation of void 21. Consequently, there is a coalescing of vacancies in the aluminum grain in the high mechanical stress field. Temperatures common in fabrication processes also aggregate the voiding problem. If a voiding problem occurs due to stress migration, electromigration effects will be accelerated in the void location due to the higher current density under operating conditions.
In one attempt to eliminate void formation, the aluminum is mixed with another metal to form an aluminum alloy. For example, copper has been added to aluminum. In turn, the copper appears to increase the energy required to cause the voids to form in the line. This remedy, however, is only partial since void formation still occurs over time, especially as the size of the aluminum line decreases.
What is needed in the art is an effective method and structure to prevent void formation due to stress migration, electromigration, and related problems.